TABLE 1.
Detection of clinical isolates producing NDM-1 carbapenemase using the modified Hodge test and the Triton Hodge test
| Species | Isolate | Acquired β-lactamase(s) | MIC (μg/ml)a |
Assay resultb |
|||||
|---|---|---|---|---|---|---|---|---|---|
| MHT |
THT |
||||||||
| IMP | MEM | ERT | MEM | ERT | MEM | ERT | |||
| Acinetobacter baumannii (n = 3) | 17042 | NDM-1 | ≥16 | ≥16 | ND | − | − | + | + |
| 17232 | NDM-1 | ≥16 | ≥16 | ND | − | + (weak)c | + | + | |
| 17575 | NDM-1 | ≥16 | ≥16 | ND | + (weak) | + | + | + | |
| Acinetobacter pittii (n = 2) | 15274 | NDM-1 | ≥16 | ≥16 | ≥2 | − | − | + | + |
| 15373 | NDM-1 | ≥16 | ≥16 | ≥2 | − | − | + | + | |
| Citrobacter amalonaticus (n = 1) | 19108 | NDM-1 | ≥16 | 8 | ≥2 | − | − | + | + |
| Citrobacter braakii (n = 1) | 19329 | NDM-1 | ≥16 | ≥16 | ≥2 | − | − | + | + |
| Citrobacter freundii (n = 3) | 15375 | NDM-1 + CTX-M-15 | 8 | 2 | ≥2 | − | + (weak) | + | + |
| 17571 | NDM-1 + CTX-M-15 | 2 | 8 | ≥2 | + (weak) | + | + | + | |
| 17572 | NDM-1 | 8 | ≥16 | ≥2 | + (weak) | + | + | + | |
| Enterobacter aerogenes (n = 1) | 17568 | NDM-1 | 8 | 8 | ≥2 | − | + | + | + |
| E. cloacae (n = 3) | 17464 | NDM-1 | ≥16 | ≥16 | ≥2 | + (weak) | + (weak) | + | + |
| 17581 | NDM-1 + PER-2 | ≥16 | ≥16 | ≥2 | − | − | + | + | |
| 19074 | NDM-1 + CTX-M-2 | ≥16 | 8 | ≥2 | − | − | + | + | |
| E. coli (n = 6) | 15792 | NDM-1 + CTX-M-15 | 2 | 4 | ≥2 | + | + | + | + |
| 17386 | NDM-1 | ≥16 | ≥16 | ≥2 | − | − | + | + | |
| 17574 | NDM-1 + CMY-2 | 2 | ≥16 | ≥2 | + (weak) | + (weak) | + | + | |
| 17758 | NDM-1 | ≥16 | ≥16 | ≥2 | − | − | + | + | |
| 19269 | NDM-1 | 4 | ≥16 | ≥2 | − | − | + | + | |
| 19426 | NDM-1 | 8 | 8 | ≥2 | − | − | + | + | |
| K. pneumoniae (n = 6) | 13717 | NDM-1 + CTX-M-15 | 4 | 4 | ≥2 | − | − | + | + |
| 17047 | NDM-1 + CTX-M-2 | ≥16 | ≥16 | ≥2 | − | + (weak) | + | + | |
| 17277 | NDM-1 + ESBLd | ≥16 | ≥16 | ≥2 | − | + (weak) | + (weak) | + | |
| 17579 | NDM-1 + PER-2 | ≥16 | ≥16 | ≥2 | + (weak) | + (weak) | + | + | |
| 17619 | NDM-1 + CTX-M-15 | 4 | ≥16 | ≥2 | − | − | + | + | |
| 17624 | NDM-1 + CTX-M-15 | 4 | ≥16 | ≥2 | − | − | + | + | |
| Morganella morganii (n = 2) | 17569 | NDM-1 | 4 | 8 | 1 | − | − | + | + |
| 17570 | NDM-1 | 8 | 8 | 1 | − | − | + | + | |
| P. rettgeri (n = 7) | 15758 | NDM-1 | 8 | 8 | ≥2 | − | − | − | + (weak) |
| 15973 | NDM-1 + PER-2 | ≥16 | 16 | ≥2 | − | − | + | + | |
| 17154 | NDM-1 | ≥16 | 8 | ≥2 | − | − | + | + | |
| 17156 | NDM-1 | ≥16 | ≥16 | ≥2 | − | − | + | + | |
| 17159 | NDM-1 | ≥16 | 8 | ≥2 | − | − | + | + | |
| 17560 | NDM-1 | ≥16 | ≥16 | ≥2 | − | − | + | + | |
| 17561 | NDM-1 + PER-2 | ≥16 | ≥16 | 1 | − | − | + | + | |
| Providencia stuartii (n = 4) | 17600 | NDM-1 | 8 | 4 | 0.5 | − | − | − | + (weak) |
| 17617 | NDM-1 | ≥16 | 2 | 1 | − | − | + | + | |
| 17638 | NDM-1 | 8 | 2 | ≤0.5 | + (weak) | + (weak) | + | + | |
| 17687 | NDM-1 + PER-2 | ≥16 | ≥16 | ≥2 | − | − | − | + (weak) | |
| S. marcescens (n = 1) | 17468 | NDM-1 | 8 | 8 | ≥2 | − | − | + | + |
IMP, imipenem; MEM, meropenem; ERT, ertapenem; ND, not determined; ESBL, extended-spectrum β-lactamase.
The overall results were as follows: MHT, meropenem, 8 positive isolates/40 total isolates (20%); ertapenem, 13 positive isolates/40 total isolates (32.5%); THT, meropenem, 37 positive isolates/40 total isolates (92.5%); ertapenem, 40 positive isolates/40 total isolates (100%).
An area of enhanced growth of the indicator strain of ≤3 mm was categorized as a weak positive result for the indicated assay.
A strain with synergism between aztreonam and amoxicillin-clavulanate disks (phenotypic test indicating ESBL production) but negative results of PCRs targeting the usual ESBL genes.